Hollow plastic container, such as plastic bottles, do not show gas barrier properties which are sufficient for the intended use in bottles. Gases such as carbon dioxide may diffuse into or out of the container. Mostly, this effect is unwanted such, inter alia, it causes a shortening of the shelf life of beverages stored in those containers.
In order to eliminate these disadvantages of plastic containers with their otherwise manifold advantages, techniques for depositing barrier layers and diffusion blocking layers respectively have been developed. In order to arrange for reducing the permeation of gases and liquids as well as to protect the plastic material against chemical corrosion or UV radiation, is it advantageous to provide substrate materials such as three-dimensional hollow bodies, with a barrier layer. By adding coatings to low-cost mass synthetics, the same barrier properties can be achieved in container walls as with expensive special synthetics, and also glass can be replaced for example in the field of pharmaceutical packagings by using this method.
The chemical vapor deposition (CVD) is a particular effective and cost-saving technology for depositing such coatings or layers. In the CVD method, a reactive chemical gas composition, which surrounds the surface to be coated is used to deposit a layer onto the surface. Inter alia, oxide layers such as SiOx layers, have proven their value as diffusion barriers.
A chemical reactive gas composition for the CVD coating can be formed by means of energyzing thermally or by ionisation of the process gases. Since synthetics normally are thermally not sufficiently stable or have a low softening temperature, the CVD coating, under high temperatures, is not suitable for coating of plastic surfaces. But here, the option of plasma enhanced CVD coating (PECVD) lends itself. Since a heating of the surface to be coated takes place here too, particularly the plasma impulse CVD coating (PICVD) is appropriate to deposit coatings on temperature-sensitive materials such as synthetics.
But current SiOx barrier layers deposited on synthetics by means of PECVD normally have a low resistance to leaches. As shown by measurements, the barrier layer is etched off almost laminarly already starting from pH-values greater than or equal to 5 in conjunction with certain ion concentrations or electric conductances of a liquid. At this, still mineral water with low CO2 content and tap water prove to be more critical concerning the stability of the layer than water pressed through osmotic filters or VE-water.
The process of etching off takes place the faster the higher the pH-value and/or the filling and storing temperature of the filled product is. Therefore, such layers have the disadvantage of having a highly reduced barrier effect, which depends on the pH-value and the storing and filling conditions, respectively, of the coating after storing a corresponding liquid in a plastic container with such a barrier layer between the plastic and the liquid. It may even happen that there is not any barrier effect at all.
The pH-value of some beverages, such as green tea, coffee produces and milk produces, as well as particularly still or only lightly carbonized mineral water, is in the range between 6.5 and 7.5. Here, a pH resistant barrier coating with a durability from 6 to 12 months at room temperature (23° C.) is requested for achieving a long shelf-life in plastic packagings.
In some countries, certain products such as green tea or coffee, are filled hotly, i.e. at temperatures of up to 95° C., subsequently stored at room temperature for up to six months, and then stored at 60° C. for up to 14 days to be sold. It also develops to heat these products by microwaves shortly before consumption. This is applied in drink dispensers for example. Also with these conditions, there is the requirement of providing a pH-resistant barrier coating.
For pharmaceutical packagings, a pH resistance of up to 10 for corresponding barrier layers is requested in case of a period of storage of typically three to five years at room temperature. Accelerated tests simulate this with a period of storage of six months at 60° C. The requirements concerning technical packagings for the packaging of, for example fine chemicals, brake fluid, cleaning agents etc. approximate the requirements concerning pharmaceutical packagings.